This invention relates generally to strain gage testing and specifically to an apparatus for calibrating strain gages at high temperatures.
An established method of calibrating and evaluating strain gages is to mount the gage on a standard test specimen of material whose characteristics are well documented and to thereby check the gage readings against predicted results based on calculations of the known characteristics of the material. Most test fixtures limit the mode of test of the standard specimen to tension for dead load tests and to bending for constant deflection tests. The dead load test is one in which the standard specimen is stressed in tension by adding weights to a fixture which causes elongation. The predictable elongation of the standard specimen is then compared to the reading taken from the strain gage to establish a calibration curve for the gage. Under the typical system of tension calibration, however, the gage calibration cannot be directly compared to results attained with an identical gage tested in a bending mode.
It is also of considerable interest to calibrate strain gages in what is termed a constant deflection mode. This is particularly of interest in systems where the gages and test specimens are subjected to varying temperatures. In such calibration system the standard specimen is deflected a fixed amount in a bending mode, usually by an inclined plane wedge, and both the specimen and the strain gage are subjected to heat. The known characteristics of expansion and other thermal effects of the standard specimen can be appropriately discounted in calibrating the changes of strain gage readings with thermal changes.
Since conventional tension testing requires a different specimen than constant deflection testing, errors are caused by the difference in strain patterns which result from different standard specimen configurations. Moreover, the different standard specimen configuration requires twice as many standard specimens to be used and requires different test fixtures for each test.
It is therefore an object of the present invention to provide an improved test fixture capable of subjecting a single test specimen to both dead weight and constant deflection testing. Another object of the invention is to facilitate the thermal cycling of the test specimen and strain gage while subjecting the test specimen to alternate testing in the dead weight and constant deflection mode. Still another object of the invention is to provide a direct means of measuring the amount of deflection to which the specimen is subjected in the constant deflection mode.